As is known, urea is produced industrially using processes based on the high-temperature, high-pressure reaction of carbon dioxide and ammonia to form ammonium carbamate, and on the subsequent decomposition reaction of the ammonium carbamate to form urea and water.
In a typical urea plant (i.e., urea production plant), these processes are normally conducted in a synthesis section comprising a urea synthesis reactor; and the urea and water solution produced in the synthesis section is then concentrated gradually—the unconverted reactants being recovered in one or more recovery sections—and is solidified in follow-up finishing (e.g., prilling, granulating, . . . ) sections.
In one known configuration, the reactants, which are recirculated to the synthesis section, are recovered in a quantity or number of sections, and more specifically in a high-pressure, medium-pressure, and low-pressure section.
A urea production plant normally produces gas streams containing unconverted reactants, including non-reacted ammonia, and so produces vent gases containing ammonia.
More specifically, the medium-pressure section vents continuous gas streams containing small amounts of ammonia and hydrogen, as well as methane and inerts (mainly nitrogen). Depending on the process used in the urea production plant, these gas streams may also contain oxygen.
The gas streams are normally mixed to conform with safety regulations, and more specifically to remain outside explosion limits.
The ammonia in the gas streams produced in the urea plant, particularly the medium-pressure section, must therefore be removed before releasing the gas streams into the atmosphere.
One fairly commonly used system for preventing ammonia emission into the atmosphere employs a torch, into which the gases (ammonia, methane, oxygen, hydrogen and inerts) from the medium-pressure section (or urea plant in general) are fed continuously. The gases are fed to the end of the torch, where a gas- (e.g., methane-) fuelled flame burns the ammonia in uncontrolled manner.
PCT Patent Application No. WO2012134288 describes a system based on the use of an incinerator, where a gas stream, rich in ammonia and hydrogen and coming from a urea production process not requiring passivating oxygen, is burned, producing a small amount of NOx; and the ammonia content is further removed, if necessary, using selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) processes.
Known systems have various drawbacks, and in particular:                the formation of large amounts of nitric oxide (NOx), in the case of uncontrolled ammonia combustion, with the risk of emitting NOx into the atmosphere;        relatively high cost, due to the continuous fuel (e.g., methane) supply necessary to keep the torch alight, and to the SCR catalyst for removing the nitric oxides produced;        the relative complexity of the process as a whole, operating with no passivating oxygen and with the need for an incinerator.        